Synthesis of myosin isoforms and their incorporation into myofibrils affects cardiac function. The goal of the proposed research is to answer the following hypotheses: I)The 3'UTR of alpha and beta myosin mRNA regulates their intracellular localization during changes in contractile activity 2)Binding of mRNPs to the 3'UTR is regulated by contractile activity. To test the function of the 3'UTR, it is necessary to make constructs that isolate this mRNA sequence from the rest of the myosin mRNA. This requires fusion constructs that attach the hypothetical regulatory 3'UTR to sequences coding for a reporter protein. Behavior of the reporter mRNA and translation of the reporter protein in transfected myocytes then give information about the function of the myosin 3'UTR. 3'UTR regulation of mRNA localization is likely to be mediated by mRNA binding proteins. Gel retardation assays coupled with UV crosslinking assays will be used to investigate mRNP binding to 3'UTR sequences. The use of antisense oligonuclotides will determine which sequences are important for mRNA localization and mRNP binding.